Nitrogen

Nitrogen and Soil The most limiting essential element in the environment Surface soil range: 0.02 to 0.5% 0.15% is representative 1 hectare = 3.3 Mg Atmosphere = 300,000 Mg as N2

Biological/Plant Nitrogen Component of living systems Amino acids Proteins Enzymes Nucleic acids (DNA) Chlorophyll Strongly limiting in the Environment

Deficiency Chlorosis – pale, yellow-green appearance primarily in older tissues.

Excess Enhanced vegetative growth – lodging Over production of foliage high in N Delayed maturity Degraded fruit quality

Distribution/Cycling Atmosphere Soil / soil O.M. Plants, animals N2, NO, N2O NH4+, NO3-, R – NH2 Proteins, amino acids Organic Nitrogen (plant tissue, Soil Organic Matter): R – NH2 During organic decomposition, R – NH2 is usually broken down to NH4+ NH4+ is converted to NO3- by soil microorganisms

Forms: mineral and organic Organic: plant/tissue N R-NH2 Mineral: soil N NH4+, NO3- Cycling in the Environment Mineralization: Decomposition of organic forms releasing nitrogen into the soil, generally as NH4+ Immobilization: Plant uptake of mineral nitrogen, removing it from the soil and incorporating into plant tissue.

Ammonium R – NH2 NH4+ NH4+ or NO3- R – NH2 Mineralization organic mineral Immobilization NH4+ or NO3- R – NH2

Organic and Mineral Nitrogen Organic R – NH2 Mineral NH4+ , NO3- mineralization immobilization } NH4+ Available to plants 1-2% generally available

Nitrogen in the Soil NH4+ NO3- Plant uptake Volatilization Leaching Nitrification Adsorption/Fixation NO3-

Adsorption and Fixation NH4+ NH4+ Clay Colloid Or Organic matter Exchange NH4+ NH4+ NH4+ Fixation

Nitrification NH4+ NO2- NO3- Aerobic (oxygen) Anaerobic (low oxygen) Nitrosomonas NO2- nitrobacter NO3-

Nitrate NH4+ NO3- NO2- NO3- Leaching to groundwater, surface water Nitrosomonas NO2- nitrobacter NO3- (to plants) NO3- Leaching to groundwater, surface water Loss of productivity Environmental hazard

Environmental Impact NO3- NO2- Methemoglobinemia – reduction by bacteria in ruminants and infants NO3- NO2- Eutrophication – stimulation of algal growth, depletion of oxygen

Eutrophication Nitrogen Photosynthetic life O2 bacteria They are rich in plant nutrients and thus their productivity is high. They produce high numbers of phytoplankton (suspended algae) which often cloud the water so that we have poor Secchi disk readings (average about 2.5 meters or 8.0 feet). These lakes also produce high numbers of zooplankton and minnows and other small fish that feed on the zooplankton. These small fish in turn provide food for the growth of larger fish. All in all, there is a high production of organic matter, like corn planted in rich soil. Much of this organic matter drifts to the bottom and forms a considerable depth of organic sediment. This sediment in turn provides the food for high numbers of bacteria. The descending plankton and the bacteria, through their respiration, can use up much or all of the oxygen from the lower depths of these lakes. Thus, one characteristic of eutrophic lakes is the summertime depletion of oxygen from the lower waters (below the thermocline – usually below about 5.5 meters or 18 feet during the summer months). Because of all of the phytoplankton produced, the eutrophic lake often has chlorophyll concentrations averaging about 14 mg/m3 or higher. The phosphorus concentration averages something over 80 mg/m3. Eutrophic lakes are often relatively shallow and often have weed beds. The weed beds are common because of the availability of nutrients and light to the shallow portions of these lakes, but also because the accumulated organic sediments provide the "soil" for their roots. Fishing is often quite good in eutrophic lakes; the high productivity of plankton and benthic (bottom) organisms in the shallows provide for relatively high numbers of fish with relatively good growth rates. Most of Michigan's eutrophic lakes are in the lower two-thirds of the Lower Peninsula. bacteria

Unimpacted N-impacted

Control Split applications Nitrification inhibitors Slow release fertilizers Cover crops Encourage natural N fixation NH4+ Nitrosomonas NO2- nitrobacter NO3-

Nitrogen and Soil Atmosphere = 300,000 Mg as N2 The most limiting essential element in the environment Surface soil range: 0.02 to 0.5% 0.15% is representative 1 hectare = 3.3 Mg Atmosphere = 300,000 Mg as N2

Symbiotic Biological Nitrogen Fixation

Rhizobium Symbiotic Biological Nitrogen Fixation Symbiosis between plant roots and rhizobium bacteria Rhizobium

Atmospheric Nitrogen Fixation N2 + 6H2 2NH3 Fixing N2 is “expensive” N N Triple bond Must use energy to break these bonds Haber - Bosch Process - Artificial Fixation of Nitrogen Gas: 200 atm 400-500 oC no oxygen yield of 10-20% Produces 500 million tons of artificial N fertilizer per year. 1% of the world's energy supply is used for it Sustains roughly 40% of the population

Area to colonize = meristem Avenue of colonization = infection thread Symbiotic Biological Nitrogen Fixation Initialization Plant roots send out signals inviting rhizobia to colonize the root Rhizobia signal plants to produce an area in which they can colonize The root produces the area and the avenue to colonize Area to colonize = meristem Avenue of colonization = infection thread N2 + 6H2 2NH3

Legume root hairs. Chemical signals sent from Root hairs to bacteria: invitation to infect

Beginning of infection Rhizobium Root hair Infection thread penetrates root cortex Root hair

Root hair curls: Traps bacteria

Nodules on legumes Packed with N2-fixing bacteria

Bacteroids in nodules Bacteroids: Rhizobium in Nodules Cells are Fixing atmospheric Nitrogen

Residue from legume crops is usually high in N when compared with residue from other crops and can be a major source of N for crops that follow legumes in rotation. Most of the N contained in crop residue is not available to plants until microbes decompose the plant material. Residues from legume crops have low carbon to N (C:N) ratios and are easily decomposed by soil microbes. Residues from non-legume crops have a higher C:N ratio and are slower to decompose N Contributions alfalfa range from 100 to 150 lbN/acre Soybeans range from 20-40 lb/acre